Why aren't sea launches used more often?

Question

I was reading yet another awesome Tildal answer, and this picture caught my eye:

   

   ^{Zenit-3SL carrying Atlantic Bird 7 lifts off from Odyssey platform (Source: The Way Up!)}

It seems that launching a vehicle directly over water would have several benefits:

1. You could launch from that 70% of the world that isn't solid land
2. You don't need to build a launch site
3. You just let the water take care of the cooling part
4. You don't need to perform nearly as much maintenance on your 'launchpad' afterward.

It seems that one potential problem would be the stability of such a surface - but aren't rockets able to make corrections in-flight for being launch a degree or so off? You wouldn't launch during the yearly tsunami, of course.

So, why aren't sea launches used more often?

Answer 1

1) Startup cost. Buying and converting a boat is much more complicated and time consuming than pouring some concrete and welding some steel.

2) Logistics. You will need at least 2 boats. One as the launcher and one as the command centre. You also need to have a reliable comms link to the rocket. You also need to house the personnel and maintain both boats. Both these costs are higher than on land.

3) Stability. Yes rockets can correct for small errors but a rolling sea is not conducive to doing precision work. A good example is Aircraft Carrier landings. The pilots are compensating for the pitching deck and are always adjusting their exact landing time. This is not good for rocket launches

4) The use of water on rocket pads is less cooling, more shockwave reduction. The ocean as a whole is a nice flat plane to bounce shockwaves off. It won't be as severe as concrete, but you would still need the spray systems.

So the concept is very sound, but by the time you get into the engineering of it, it is many times easier to launch from land.

Answer 2

The major reasons for Sea Launch's approach is to be able to launch on the equator, and not to have to worry about overflying any territory, where stages might drop.

The cooling aspect is probably the least interesting part.

But the main argument against it is the infrastructure costs, and salt water corrodes everything, it would seem. No matter how well you protect against salt water, it gets all over, when you are out in the ocean.